Under the Hood: LifeLong's FM Radio tunes in low-cost analog ASICs

Go back far enough and you had to cough up some pretty serious coin for a boxy little FM receiver with built-in speaker and telescoping antenna. But the early years of radio design have long since given way to the era of mass-produced gadgetry, and the current technology is what makes for a story.

As an example of how inexpensive FM radios have become, we look here at a near-brandless made-in-China FM radio unit purchased at a local drugstore"with headphones" for $2.99. If you are looking to splurge, the same unit was marked with a two-for-$5 price tag, presumably with a touch of profit still left in the mix. Trade show junkies may have even picked up a similar radio as a company booth giveaway.

How did they get to be so cheap? In part, low-cost technology manufacturing economies have driven down costs, but the real enabler in this case is mixed-signal analog IC technology. Central to this radio's electronics is the Philips TDA7088 (mono) FM receiver circuit, itself a somewhat old-school bipolar chip design dating back to the mid-90s. Typical of IC-based radio chips, the TDA7088 supports single button tuning which allows a scan-and-stop function at available channels with a reset button to start the search over at the low end of the radio band. An internal circuit of the TDA7088 generates a ramp voltage across an external varicap diode" whose capacitance changes in proportion to reverse diode bias"to affect tuning. A handful of passives, along with a volume control potentiometer and a two-transistor discrete audio amplifier circuit, round out system components.

To avoid a bulky telescoping antenna, the FM signal is picked up in the earbud cable and separated from the audio signal with a simple highpass circuit between headphone jack and FM antenna input on the TDA7088.

Newer generations of radio chips have of course advanced the single-chip FM radio paradigm, offering stereo, integrated amplifiers, and a host of other features. Such devices are increasingly finding their way into cell phones to provide for tunes during those idle moments between phone calls.

The radio case shows the item to be called "FM Digital"" ironic since the only digital technology involved is your finger to manipulate the controls. Other than that, exceptionally inexpensive analog ASICs deliver the goods.

David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)